This invention pertains to a new type of high performance polymer membranes derived from aromatic polyimide membranes and methods for making and using these membranes.
In the past 30-35 years, the state of the art of polymer membrane-based gas separation processes has evolved rapidly. Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Membrane gas separation is of special interest to petroleum producers and refiners, chemical companies, and industrial gas suppliers. Several applications have achieved commercial success, including carbon dioxide removal from natural gas and from biogas and enhanced oil recovery, and also in hydrogen removal from nitrogen, methane, and argon in ammonia purge gas streams. For example, UOP's Separex™ cellulose acetate polymeric membrane is currently an international market leader for carbon dioxide removal from natural gas.
The membranes most commonly used in commercial gas separation applications are polymeric and nonporous. Separation is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the membrane polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
Polymers provide a range of properties including low cost, permeability, mechanical stability, and ease of processability that are important for gas separation. A polymer material with a high glass-transition temperature (Tg), high melting point, and high crystallinity is preferred. Glassy polymers (i.e., polymers at temperatures below their Tg) have stiffer polymer backbones and therefore let smaller molecules such as hydrogen and helium pass through more quickly, while larger molecules such as hydrocarbons pass through more slowly as compared to polymers with less stiff backbones. However, polymers which are more permeable are generally less selective than less permeable polymers. A general trade-off has always existed between permeability and selectivity (the so-called polymer upper bound limit). Over the past 30 years, substantial research effort has been directed to overcoming the limits imposed by this upper bound. Various polymers and techniques have been used, but without much success. In addition, traditional polymer membranes also have limitations in terms of thermal stability and contaminant resistance.
Cellulose acetate (CA) glassy polymer membranes are used extensively in gas separation. Currently, such CA membranes are used for natural gas upgrading, including the removal of carbon dioxide. Although CA membranes have many advantages, they are limited in a number of properties including selectivity, permeability, as well as chemical, thermal, and mechanical stability. For example, UOP's early field practice found that polymer membrane performance can deteriorate quickly. The primary cause of loss of membrane performance is liquid condensation on the membrane surface. Condensation is prevented by providing a sufficient dew point margin for operation, based on the calculated dew point of the membrane product gas. UOP's MemGuard™ system, a regenerable adsorbent system that uses molecular sieves, was developed to remove water as well as heavy hydrocarbons from the natural gas stream, hence, to lower the dew point of the stream. The selective removal of heavy hydrocarbons by a pretreatment system can significantly improve the performance of the membranes. Although these pretreatment systems can effectively remove heavy hydrocarbons from natural gas streams to lower their dew point, the cost is quite significant. Some projects showed that the cost of the pretreatment system was as high as 10 to 40% of the total cost (pretreatment system and membrane system) depending on the feed composition. Reduction of the pretreatment system cost or total elimination of the pretreatment system would significantly reduce the membrane system cost for natural gas upgrading. On the other hand, in recent years, more and more membrane systems have been applied to large offshore natural gas upgrading projects. For offshore projects, the footprint is a big constraint. Hence, reduction of footprint is very important for offshore projects. The footprint of the pretreatment system is also very high at more than 10-50% of the footprint of the whole membrane system. Removal of the pretreatment system from the membrane system has great economical impact especially to offshore projects.
High performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole have been developed recently to improve membrane selectivity, permeability, and thermal stability. These polymeric membrane materials have shown promising properties for separation of gas pairs such as CO2/CH4, O2/N2, H2/CH4, and propylene/propane (C3H6/C3H8). These high performance polymeric membrane materials, however, have reached a limit in their permeability-selectivity trade-off relationship. The membranes having high permeabilities generally have low selectivities and vice versa. In addition, gas separation processes based on the use of glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer as represented by the membrane structure swelling and significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Barsema et al. reported that heat treatment of Matrimid® membranes in an inert atmosphere can alter the membrane properties as well as molecular structure. See Barsema, et al., J. MEMBR. SCI., 238: 93 (2004). These heat-treated polyimide membranes showed improved plasticization resistance. However, these heat-treated polyimide membranes did not show significant improvement in selectivity and permeability compared to the untreated polyimide membranes.
U.S. Pat. No. 4,717,393 by Hayes disclosed photochemically crosslinked aromatic polyimide membranes. The photochemically crosslinked aromatic polyimide membranes showed significantly enhanced selectivity but decreased permeability for CO2/N2 and H2/CH4 separations compared to the uncrosslinked aromatic polyimide membranes.
In U.S. Pat. No. 7,485,173 B1 by Liu, et al., it was reported that the use of UV crosslinking did succeed in improving the selectivities of certain mixed matrix membranes that contain molecular sieves that function to improve the permeability and selectivity of the membranes. However, it was necessary both to crosslink the polymer using a UV radiation source and to add the molecular sieves to obtain the improved levels of performance reported therein. None of the membranes reported in this U.S. patent exhibited CO2 permeability higher than 200 Barrer at 50° C. testing temperature for the removal of CO2 from natural gas. It is highly desired to have improved polymeric membranes that do not contain molecular sieves both to avoid the need to disperse the molecular sieves and to eliminate any problems caused by the lack of adhesion between the polymer and the molecular sieves.
The present invention provide a new type of high performance polymer membranes derived from aromatic polyimide membranes that can overcome the problems of the prior art polymer membranes. These new polymer membranes derived from aromatic polyimide membranes have both high selectivity and high permeability (or permeance), as well as high thermal stability.